Recently, the main stream of the image formation by the electrophotography is changed to digital system. In the image formation by the digital system, a small diameter toner for developing electrostatic image, hereinafter also referred to as the toner, is essential by which excellent fine line reproducibilty and high resolution can be realized since in such the system a small dot image of 1200 dpi level (dpi is dot number per inch, one inch is 2.54 cm) is developed.
As examples of producing method of such the small diameter toner, Japanese Patent Publication Open for Public Inspection, hereinafter referred to as JP O.P.I. Publication, Nos. 2002-296839 (Patent Publication 1) and 2002-351140 (Patent Publication 2) disclose a method by which materials of toner such as polyester or polyol resin and others are emulsified in an aqueous medium and particles thereof dispersed in the emulsion are aggregated.
As an embodiment of the digital image formation, an image formation method by a print-on-demand system is developed by which required number of image is printed at a demanded time. The image formation by such the system is noted as an important image forming method for replacing to usual light printing system since the plate making in the usual printing system is not necessary and publication of several hundreds copies and making of direct mails or invitation cards while changing the address are made possible by such the system.
A problem has been found, however, in the course of the printing of the mails or the invitation cards while changing the address by the electrophotographic image formation. The problem is that sufficient fixing cannot be performed when the image formation is carried out on a thick paper such as a thick post card and a social invitation card. Particularly, on a card having a gray colored frame relating funeral, the fixing at the part of gray frame is difficultly performed and the unfixed toner causes contamination of hands and paper surfaces.
Moreover, the toner particles tend to be crushed and thus formed fine toner particles cause the contamination of paper surface since a large load is applied onto the toner particles when the toner image is formed on the surface of thick paper which is not comparable to the load when the image is formed on thin paper such as paper for usual copying.
The thick paper such as the post card is one of the image receiving materials with the highest difficulty. Furthermore, it is necessary for spreading the electrophotographic system as the image forming method for the print-on-demand that the image can be stably formed on offset printing paper with high smoothness, not on the usual paper developed for the image receiving material for electrophotographic use. The method can difficultly accepted by the printing industry field without satisfaction of such the requirement.
It is frequently seen in a train that a commuter reads a paperback holding on his one hand while holding onto a trap by another hand. The printed paper is required to have slipping ability satisfactory for turning over leaves of the book by the one hand and the toner is required to have fixing strength so that the toner does not contaminate the paper and characters by friction.
However, the spreading of the method is late since the slipping ability and the fixing strength of the toner image by the electrophotographic method are inferior to those of the usual printing material and are not accepted by publishers. These problems cannot be solved by the use of the toners disclosed in the foregoing Patent Publications 1 and 2.
A problem so called toner blister that a white grain like image defect is formed at the high density portion of the toner image when the toner obtained through the washing process disclosed in Patent Publications 1 and 2. Such the phenomenon is apparent when the small particle toner having an average diameter of not more than 7.4 μm is used. Particularly, such the tendency is made notable when the printing is continuously repeated by 100,000 or more times or the printing is continuously carried out under a low temperature condition. It is supposed that the moisture contained in an impurity such as an aggregating agent remaining on the toner particle surface is changed to steam by heating in the fixing process and discharged so that such the image defect is caused by breaking of the toner layer by discharging of the steam.
For removing the aggregation agent remaining on the toner particle surface, a treatment using an acid or an acid and alkali has been tried. However, both of the methods were insufficient.
Any washing technique for completely remove the impurity from the toner particle surface by mechanical means is not established yet.
Patent Publication 1
JP O.P.I. Publication No. 2002-296839 (Paragraph 0011)
Patent Publication 2
JP O.P.I. Publication No. 2002-351140 (Paragraph 0011)